1. Field of the Invention
The present invention relates to apparatus for centering components or objects relative to a pick up tool prior to placement on a workpiece. More particularly, the present invention relates to an apparatus for optically and electronically determining the X, Y and rotational position (O--) of a component after being picked up by a pick up tool and prior to placement on a workpiece to permit correction of the position of the pick up tool to compensate for misalignment of the component thereon.
2. Description of the Prior Art
Pick and place machines for placing small electrical and electronic components on printed circuit boards and substrates are well known and are commercially available from U.S. companies such as Quad Systems Corporation in Horsham, Pa. and from Universal Instruments Corporation in Binghamton, N.Y.
Automatic component placement machines, known as pick and place machines have heretofore employed centering devices, centering chucks and end effectors which physically center or reposition the electrical and electronic components on the end of a pick up tool prior to placement on a workpiece. Such mechanical devices are typical of those shown in Quad U.S. Pat. No. 4,987,676 and Universal U.S. Pat. No. 4,135,630 which employ centering fingers to physically move the components on the end of the pick up tool to a desired predetermined position prior to placement.
Electronic equipment is being miniaturized to increase operating speeds as well as to effect reliability and reduce cost. Typical resistors, capacitors and other discrete components are now so small that they can no longer be accurately positioned or repositioned on a pick up tool by physically moving the component. Some components and semiconductor devices have electronic terminals or leads on their sides, as well as the bottom and top surfaces which further prevent physically moving and realigning the component on a pick up tool.
It is well known that robotic arms which are capable of multiple axes movement and placement of objects have backlash errors which decrease the accuracy of placement of the object or component. To overcome the inherent backlash error in such systems, the robotic arm is moved to a positioning station at a known predetermined position near the work piece. Either the object and/or the robotic arm is then sensed by various forms of remote sensors at the scanning or sensing station without physically touching the robotic arm. The sensor determines the deviation of the robotic arm between its desired position and from the known position and the error is used to calculate the final movement of the robotic arm to place the object or component at a more accurate desired position.
U.S. Pat. No. 4,615,093 shows an adaptation of a position scanning station placed intermediate a component pick up station and a work piece. The scanning station employs laser beams to effect reflection or shadowing position techniques as well as using mechanical scanning.
The prior art scanning stations depicted in this U.S. Pat. No. 4,615,093 and references cited therein are expensive and slow down the speed of operation of a pick and place machine which must be moved to the scanning station to effect the scanning and repositioning calculation prior to placement at a work piece for correction of component presentation errors.